The invention relates to an optical switching device. More particularly, the invention relates to an optically-controlled optical switching device.
Following the extensive development of optical fibre transmission systems, there is great awareness of the demand for devices capable of high-speed execution of different operations on the bits which constitute the digital information of an optical signal.
The current equipment used for processing the optical signals is inadequate for managing the ever-increasing transfer speeds possible in optical fibre transmission systems. In point of fact, equipment of this type consists of digital electronic devices with a limited band with respect to the optical band available in optical fibre transmission systems, and is based on a serial processing of information.
Hence, there is still great awareness of the need to make full use of the optical band available in optical fibre transmission systems by keeping the signal in an optical form, thus avoiding opto-electronic and electro-optical conversions.
Moreover, some compounds of a crystalline nature are known which are capable of transmitting polarized light onto a plane which, in given conditions, can be made to rotate. Examples of compounds of this type are some liquid crystals, e.g. CdTe:In, Bi.sub.12 SiO.sub.20 and others.
In particular, CdTe:In becomes birefringent under the action of an electrical field, and the indices of refraction of the material vary according to variations in the intensity of the said field. A second property of this material is that, when the electrical field applied remains constant, the indices of refraction can be varied subsequently under the action of light.
The first of the aforementioned effects is commonly called the "electro-optical effect", and the second, the "photoconductive effect".
These properties of CdTe:In have been much investigated, and the hypothesis with the greatest credence at present is that an optical beam incident in a zone of a CdTe:In crystal subjected to a constant electrical field excites charge carriers in the conduction band from the impurity levels. Under the action of the said applied electrical field, these photogenerated charge carriers migrate into the adjacent dark region (not illuminated by the optical beam) where they are trapped.
The resultant spatial-charge density associated with the trapped charge carriers creates an opposite electrical field to the one applied.
At suitable intensities of the optical beam, the electrical field generated balances, in the illuminated zone, the one applied.
The effect thus created in the illuminated zone is called the "shielding effect". Hence, a polarized optical signal propagated in the illuminated zone of the optical control beam is guided by indices of refraction differing from those of the crystal to which only the electrical field has been applied.
On the other hand, given that the voltage along the crystal is constant, the reduction (or elimination) of the electrical field in the illuminated zone brings about an increase in the electrical field in the dark zone of the crystal (the enrichment effect) and a corresponding change in the indices of refraction of the material. Hence a polarized optical signal propagated in the dark (enriched) zone below or above the optical control beam travels on a plane of polarization different from that on which it would be propagated in the absence of the control beam, and different from that on which it would be propagated in the zone illuminated (or shielded) by the control beam.
William H. Steier et al ["Infrared power limiting and self-switching in CdTe", Appl. Phys. Lett., 53 (10), 840-841, (1988)] describe a power limiter and a "self-switch" which use the shielding effect of the electrical field generated by the photocharges created owing to the photoconductivity of CdTe:In at 1.06 .mu.m. The devices described by these Authors utilize a single incoming optical beam which, with an increase in intensity, causes the shielding effect and allows the said beam to behave simultaneously like a signal and like a control beam.
William H. Steier et al ["Opto-optical switching in the infrared using CdTe", Optics Letters, 14 (4), 224-226, (1989)] describe a CdTe:In opto-optical switch for a signal with a wavelength of 1.06 .mu.m and an optical control beam with a wavelength of 1.06 .mu.m, which makes use of the photocharge generated by the optical control beam and the resultant shielding effect of the electrical field. However, the Authors have not taken into consideration the possibility of simultaneously propagating more independent polarized optical signals and the relevant collinear and superimposed control beams in the zone affected by the shielding effect of a single crystal.
Ziari M. Et al ["Infrared nonlinear neurons using the field shielding effect in CdTe", Applied Optics, 29 (14), 2074-2083, (1990)] describe an IR ray neuron which utilizes the shielding effect in CdTe:In, in which a single optical actuating (or control) beam, transverse or collinear with respect to the polarized optical signal, creates a spatial charge field dependent upon the intensity of the said beam.
Moreover, the Authors have utilized the birefringence caused by the electro-optical effect, i.e. the variation in the indices of refraction of the CdTe:In, in a basic one-neuron configuration, demonstrating that at high intensities of the actuating optical beam there is a nonlinear saturated response, with absorption losses less than or equal to 1.0 dB. Studies have also been made of both the functioning of the neuron with synchronous control pulses lasting microseconds, and the availability of both the inhibiting inputs (enriching effect) and the excitation inputs (shielding effect). Finally, other characteristics were discussed, including the option of a broadband response (0.9-1.4 .mu.m) with incoherent inputs and a configuration for a bi-directional neuron which can be used in devices which learn from the counterpropagating error.
On page 2076, left-hand column, lines 4-7, the Authors assert that it is not necessary for the control beams originating from a plurality of sources to be accurately aligned. From this one is led to deduce that, in this experiment, the "shielding effect" caused by an optical control beam involves the entire crystal, and it is thus not possible to control independently of one other and in a single crystal, different polarized optical signals, by means of a plurality of control light beams.
Liu L. et al ["Logic gate modules using opto-optical birefringence switching", Optics Letters, 16 (18), 1439-1441, 1991] describe the use of a birefringence phenomenon to construct logic gates which output three logical functions. The logic gate modules described by the Authors consist of Bi.sub.12 SiO.sub.20 opto-optical birefringent switches in which the polarized optical signal is propagated in the dark zone affected by the enrichment effect, of LiNbO.sub.3 electro-optical half-wave plates, and of calcite plates. Each logic gate can be guided by optical control inputs parallel to the polarized optical signal, and electrically programmed to execute different logic operations. However, the Authors have never taken into account the possibility of simultaneously propagating a plurality of independent polarized optical signals and the relevant collinear and superimposed control beams in the zone affected by the shielding effect of a single crystal.
Liu L. et al ["Opto-optical switching using field enhancing effect in Bi.sub.12 SiO.sub.20 ", Journal of Applied Physics, 72 (2), 337-343, 1992] describe a mathematical model of a Bi.sub.12 SiO.sub.20 opto-optical switch which makes use of the photocharge created by the control beam, parallel to the polarized optical signal, and the resultant enrichment of the electrical field in the dark zones. The Authors analyze the enrichment effect of the electrical field and the electro-optical modulation with both the PocKels effect and the optical activity. On the basis of their mathematical treatment, the Authors set out various considerations regarding the design of the switches.
Ziari M. Et al ["Optical switching in cadmium telluride using a light-induced electrode nonlinearity", Applied Optics, 32 (29), 5711-5723, 1993] set out a theory and describe experimental results concerning the growth and erasure of an electrical field in the region just below the CdTe:In negative electrode as a function of the wavelength of optical control beams which are incident on the transparent negative electrode and propagate in the crystal perpendicularly to the direction of the polarized optical signals. The light (850-920 nm) absorbed by the impurities below the band gap causes the growth of a region with a very high electrical field (E&gt;&gt;20 kV/cm) just under the negative electrode. Whilst the illumination has wavelengths above or in the region of (800-840 nm) the band gap, it can erase the high electrical field. The writing and erasing of the field depend upon the illumination and the Authors foresee its use, when combined with the electro-optical and electro-absorption effects, to produce one-dimensional infrared spatial modulators with polarized optical signal beams in the 900-1500 nm range which nonetheless use a totally different effect from the shielding effect (page 5715, right-hand column, lines 24-35). Moreover, they hypothesize the use of these to produce two-dimensional infrared spatial modulators. In this case, the control beams and the polarized optical signals would both propagate across transparent electrodes and along the direction of the electrical field applied (page 5719, right-hand column, lines 52-55).
Boffi P. et al ["Photonic sampler for 1550-nm signals", Optics Letter, 20 (6), 641-643, 1995] describe a device which extracts samples of configurable length from a 1550-nm polarized optical communication signal propagated in the region affected by the shielding effect, under the control of an optical control beam transverse to the polarized optical signal. The said device is produced by means of two indium-doped cadmium telluride switches (CdTe:In) and is characterized by a rise and fall time of 10 ns and by a sampling window of one microsecond in duration.
Boffi P. et al ["Optical time-to-space converter", Optics Communications, 123, 473-476, (1996)] describe an all-optical time-space converter, produced in free propagation, which translates binary time-coded words into equivalent space-coded words with a 1550 nm polarized optical communication signal which is propagated in the region affected by the shielding effect. Conversion is effected by means of four optical gates, one for each of the four polarized optical signals. Each optical gate comprises a first and second indium-doped cadmium telluride crystal (CdTe:In). There are two optical control beams, one for the first four crystals and one for the second four crystals, which they illuminate transversely.
Boffi P. et al ("Photonic time-space converter for digital communication signals", Optical Computing, Sendai, 12-13, 1996) describe a photonic subsystem which converts the time-coded bit sequence into an equivalent space-coded figure, with a 1550 nm polarized optical communication signal which is propagated in the region affected by the shielding effect, the said subsystem being made completely optically in free propagation under the control of an optical control beam which is transverse with respect to the polarized optical signal. The device used is that already described by Boffi P. et al in "Optical time-to-space converter", Optics Communications, 123, 473-476, (1996).
Pietralunga S. et al ["CdTe:In Monocrystal Modules for All-Optical Processing", Journal of Nonlinear Optical Physics and Material, 5 (2), 247-268, 1996], as well as summarizing the preceding work of Boffi et al, describe the application of the photoconductive properties of the n doped CdTe: In electro-optical monocrystal in order to process totally optically the .lambda.=1550 nm polarized optical signal which is propagated in the region affected by the shielding effect. The Authors describe different types of CdTe:In elementary modules with free-propagation architecture: a non-coherent wavelength converter, a switch, a sampler and a time-space converter. In the case of the switch, the sampler and the time-space converter, the optical control beam is transverse to the polarized optical signal which is propagated in the zone affected by the shielding effect. In the case of the non-coherent wavelength converter, the control beam may be collinear with the polarized optical signal. Nevertheless, on page 265, lines 13-16, the Authors expressly acknowledge that studies and experiments are still necessary to establish which type of illumination is most suitable to sample a plurality of polarized optical signals in a single crystal.
The state of the art thus suggests that the "shielding effect" caused by an optical control beam would involve the entire crystal, and it would hence not be possible to control various polarized optical signals in a single crystal independently of one another via a plurality of control light beams.